1. Ulcers in the interstine are difficult to heal. IMHO, its wrong. May be your GI's idea makes sense to a layman's mind but my scientist's mind fails to buy it. Pepetic ulcers are healed without any issues, how so? In UC, if you look at pathophysiology, the cuase of ulcer is different, epithelial lining of your intestine is totally messed up. So colitis ulcer is not quite similar to a cut or bruise on your hand/finger. Nobody has figured it out, how to bring that messed up epithelial lining back to normal.
2. Prepping colonoscopy - Of course, you will just have water coming out. The reason, most of these medicines do only one job, increase peristaltic movement of your intestine, so that most of the solids in the intestine are out. First one or 2 rounds usually get all solids out of your intestine, all rest visit is just something like washing up the colon. If you managed to see your colonoscopy, you must have seen, it still doesn't clean up your intestine fully. There are still patches here and there.
So drinking clear fluids, in other words, will NOT help healing ulcers in anyway. Its the pathophysiology thats different. But your idea will make a perfect sense if your have peptic/gastric ulcers. Did I make my point clear?
If you still want to try it, try smoothies made using culture yoghurt.
Oh one more thing, may be people say that weight loss is risk in UC but I am UC patient for about 3.5 years now and have not lost a single kilo, true that I have changed my eating style completely, cutting out all junk and eating only when I need it rather than pleasure or timepass (like while wathicng movie etc).